1. Field of the Invention
This invention relates to an image sensor comprising a transparent substrate supporting a two dimensional array of photosensitive and switching elements and switching signal conductors and data signal conductors connected to the photosensitive and switching elements for enabling individual photosensitive elements to be accessed, the photosensitive and switching elements and conductors being arranged such that at least half of the area of the image sensor is transparent during operation.
2. Description of the Related Art
Such an image sensor is described in EP-A-378593 (International Publication No. WO 89/09960). The image sensor described in that document has a matrix array of photosensitive diodes connected back to back with switching diodes between row and column conductors. Each photosensitive diode may be formed on top of the associated switching diode. The image sensor is designed to be placed on top of, for example, an electro-optic display which requires the image sensor to be sufficiently transparent that the image produced by the display is still identifiable when viewed through the image sensor. As indicated in the document, the photosensitive diodes are designed to occupy equal to or less than 50% of the surface area of the image sensor. Clearly, the smaller the area occupied by the photosensitive diodes the better the view of the display image which can be obtained through the image sensor. However, the light collection efficiency of the photosensitive diodes reduces with area. It is accordingly necessary to accept a compromise between the dynamic range (that is the detection limit or number of gray scale levels which can be detected under a given illumination level) of the image sensor and the degree of obscuration of the display device.